Mixer-settler apparatus

ABSTRACT

Apparatus for simultaneously mixing and separating multiple phase fluids in which one phase is retained as a batch phase and the other phases are continuously flowing. A pump-mixer continuously mixes the fluids and pumps them into a settling chamber where they settle out and means are provided in the settling chamber for returning one phase to the mixer from which it came and transferring the other phases to another pump-mixer. Apparatus is provided for retaining as a batch or captive phase either heavy or light phases.

I United States Patent [1 1 3,663,1 78 Miller et al. [451 May 16, 1972541 MIXER-SETTLE]! APPARATUS 3,489,526 1 1970 El-Roy ..23/267 72Inventors: William E. Miller, Naperville; James B. 213;: 23/270 5Knighton, Joliet; George J. Bernstein, Park Forest, all of III. OTHERPUBLICATIONS g Tht Unlled s"!!! Alllfl'lcl l5 Chem Eng. Progress, Vol.50, No. 8, pp. 403-408 represented by the United States Atomic MixerSettler Extraction Equip., pp. l88- I97, Davis Jr., Energy CommissionChem. Eng. Frog, Apr. 1954 [22] Filed: June 1969 Primary Examiner-NormanYudkofl [2|] Appl. No.: 829,945 Assistant Examiner$. SilverbergAttorney-Roland A. Anderson [52] US. Cl. ..23/267 R, 23/2705, 23/310 57ABSTRACT [51] Int.Cl. ..B0li 7/00 5s Fleld of Search ..23/27|, 269, 277,310, 270.5 Apparatus for simultaneously mixing and separalms multiplephase fluids in which one phase is retained as a batch phase [56]References u and the other phases are continuously flowing. A pumpmixercontinuously mixes the fluids and pumps them into a settling UNITEDSTATES PATENTS chamber where they settle out and means are provided inthe settling chamber for returning one phase to the mixer from NOI'C" itcame and f i g the other ph to another 2'646'346 7,1958 Coplan "mu/270'spump-mixer. Apparatus is provided for retaining as a batch or 2,895,8087/1959 l-lartly.... ..23/27o.5 captive phase either heavy gm phases2,937,078 5/1960 Duhes ..23/270.5 2,701,753 2/1955 Eisenlohr ..23/310 4Claims, 5 Drawing Figures MlXER-SEI'ILER APPARATUS CONTRACTUAL ORIGIN OFTHE INVENTION The invention described herein was made in the course of,or under, a contract with the United States Atomic Energy Commission.

CROSS-REFERENCED APPLICATIONS This case is related to companion caseSer. No. 830,080 filed June 3, 1969 which is directed to a processperformed with the apparatus of this invention.

BACKGROUND OF THE INVENTION This invention relates to a process andapparatus for mixing and separating multiple phase fluids in which acontinuous feed is simultaneously mixed with and separated from multiplebatch or captive solvents.

Mixing of multiple liquids is a common phenomenon in chemicalengineering processes and often is accomplished in one of two ways,continuous countercurrent flow or batch contact. In the continuouscountercurrent type of mixing two immiscible fluids enter an elongateddevice from opposite ends, flow into each other to provide mixing andexit at the opposite end from which they entered. In batch mixing,multiple immiscible fluids are charged to a vessel and agitated.Thereafter they are allowed to settle out and are removed from thevessel by decanting or other means well known in the art. Mixing andsettling operations are usually performed in a unitary apparatus wheremultiple fluids are mixed at one point in the apparatus and allowed tosettle out in another part of the apparatus. Both the countercurrent andthe batch apparatus have advantages and disadvantages. So too, theprocesses have advantages and disadvantages, the batch processes areslow but provide for easy reuse of a particular fluid, while continuousoperations are faster they do not particularly lend themselves to reuseof the constituent fluids.

Both batch and continuous flow mixing and settling operations haveapplication to a pyrochemical separation of plutonium from uranium fromfission products. Some of the particular chemistry contained herein haspreviously been made of record in US. Pat. Nos. 3,326,673; 3,282,68l;and 3,284,190 which only contemplated batch operations. From an economicstandpoint, batch pyrochemical operations are not competitive withpresent day continuous flow aqueous processes, and for that reason.among others, it was necessary to invent a new type of separationprocess and apparatus therefor, described hereinafter with particularreference to a two-phase liquid separation but not limited thereto.

It is the principal object of this invention to provide a process andapparatus in which the primary advantages of both batch and continuousflow mixing and separating are combined into a new process and apparatustherefor.

BRIEF DESCRIPTION OF THE DRAWINGS This invention may be betterunderstood with reference to the following drawings in which:

FIG. I is a flow diagram for a representative process of this invention;

FIG. 2 is a plan view of a series of mixer-settlers having a captivelight phase;

FIG. 3 is a plan view of a series of mixer-settlers having a captiveheavy phase;

FIG. 4 is a section of FIG. 2 take along line 4--4; and

FIG. 5 is a section of FIG. 3 taken along line 5-5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The invention willfirst be explained with respect to the process and thereafter withrespect to apparatus in which the process may be performed. FIG. I is aflow diagram in which a dissolver is charged with a stainless steel cladplutonium dioxide-uranium dioxide fuel subassembly ll, shorn of majorportions of its hardware. Consecutively, streams of molten zinc 12,molten metal reduction salt 13, and molten magnesium-copper-calciumalloy 14 are charged to dissolver l0. Molten zinc will dissolvestainless steel but not affect the oxide fuel, as shown in US. Pat. No.3,567,648 issued Mar. 2, l97l to W. .l. Walsh and R. Dean Pierce. Afterdissolution of the stainless steel is complete, various volatiles andthe zinc-stainless steel solution are transferred from dissolver 10 towmte. The reduction salt 13 may be various alkali earth halides, but acombination of calcium chloride and calcium fluoride is preferred. Thecalcium in alloy 14 reduces the fuel oxides to form the respectivemetals. The plutonium present in the fuel, as well as various rareearth, noble metal and refractory metal fission products, transfers tothe magnesium-copper alloy which is now depleted in calcium. Thecalcium, initially present in stream 14, has been oxidized to calciumoxide in the reduction of the various plutonium or fission productvalues. The magnesium-copper-plutonium-fission product alloy leavesdissolver 10 as product stream 15; uranium present in subassembly 11 aswell as some fission products leave dissolver 10 in a different productstream (not shown) having a different magnesium-copper composition to bepurified in a process similar to the one hereinafter described.Dissolver I0 is operated somewhat as a batch process in that a fixednumber of subassemblies I l are dissolved at a time.

Production stream 15 is combined with a magnesiumcopper recycle stream16 and flows into a plurality of mixersettlers 17 as a continuous feedstream 18, the first mixer-settler in the series being denoted as 17a.Mixer-settlers 17 are representative of n stages, each of which has acaptive salt phase. The numerical value of n is determined ashereinafter explained. In mixer-settlers 17 the continuous feed streamI8 is simultaneously mixed with and separated from a molten salt 19 intowhich some rare earth fission products transfer. Each mixer-settlerl7a-d contains its own salt l9a-d which remains captive within therespective mixer-settler, that is the salt in each mixer-settler is abatch charge while feed I8 is a continuous stream flowing from onemixer-settler to another. As shown by the dotted lines, provision ismade for periodically transferring salt 19d to mixer-settler 17c, salt190 to mixer-settler 17b, salt 1% to mixer-settler 17a and fordischarging salt I9a as waste stream 20 for reasons and with apparatushereinafter explained. Make-up salt stream 2] introduces new salt intomixer-settler 17d after the above discharge and series of transfers havetaken place.

Mixer-settler 22 is representative of p stages, each of which is acontinuous flow countercurrent exchanger where plutonium is transferredfrom the feed alloy 18 to a salt 23 which need not be, but forconvenience is, the same composition as salts 19. Part 184 of themagnesium-copper feed alloy l8 now depleted in plutonium flows back tomixer-settler as recycle stream 16 and part flows to waste as stream16w. Salt 23, containing plutonium in the form of a chloride salt, flowsas a continuous feed into and through mixer settlers 24 and 25 which arerepresentative of the last m stages, each stage containing a captivemetal phase.

Mixer-settler 24 has as a captive phase an alloy 26 which iscontinuously and simultaneously mixed with and separated from salt feed23 and removes noble and refractory metals from the salt. Inmixer-settler 25, plutonium transfers from salt feed 23 into an acceptoralloy 27 which flows into a retorting vessel 28 where the constituentsof alloy 27 are distilled away and returned to mixer-settler 25 whileplutonium remains in the vessel and is transferred to a converter 29. Inthe converter 29, the plutonium is combined with uranium from theprocess (not shown) and converted into a mixed oxide fuel. The acceptoralloy 27 is never rejected to waste as shown by the closed loop in thefigure and only needs an occasional addition of the most electropositiveconstituent due to losses thereof incurred in the reduction of theplutonium chloride values from salt 23. Similarly, the salt 23 alsoforms a closed recirculating loop and is reusable indefinitely.

In the process as shown, the first four stages, mixer-settlers l7a-d,are employed to extract rare earth fission products into the captivesalts l9a-d; for example, lanthanum, cerium, praseodymium, neodymium,promethium, samarium, europiurn, gadolinium, terbium and dysprosium arerepresentative of those rare earth fission products which are extractedinto the salt. Yttrium is also extracted and the term "rare earths"should hereinafter be construed to include yttrium. The extraction takesplace by means of oxidation as shown in the above-referenced U. S.Patents. The number of rare-earth fission-product extraction stagesemployed in any particular process depends upon the required degree ofpurification. The use of four stages permits a decontamination factorfor rare earth fission products of ID to l0", while the use of twostages results in a decontamination factor of 10 to 10. Thedecontamination factor is the ratio of the quantity of fission productsin the subassembly 11 divided by the quantity of fission products in thefinal product. Although various salts may be used to effect thedecontamination, a mixture of magnesium chloride, various alkali metalchlorides and some magnesium fluoride is preferred.

For convenience, salts [90-11 are the same and should contain as muchmagnesium chloride as possible for effective fission product transfer.The melting point of magnesium chloride is over 700C. and alkali metalchlorides are added to reduce the melting point of the salt whilemagnesium fluoride is added to enhance the separation of the salt 19from the metal feed 18 in the mixer-settlers. The following saltcomposition is preferred: 49 mole percent (m/o) MgCl,-29 m/o NaCll9 m/oKCl3 m/o MgF A Mg-29 atom percent (a/o) Cu-34 a/o Ca alloy 14 ispreferred but there is considerable leeway which is consideredacceptable as is true for the CaC1, m/o CaF, reduction salt 13 which isthe preferred composition. The final wash alloy 26 is a Mg90 w/o Cdalloy but others such as zinc-cadmium are acceptable; the acceptor alloy27 may also be a CdMg alloy or a Cd- MgZn alloy but a ZnMg alloy ispreferred.

In summary then, n captive salt stages may be used in the processdepending upon the extent of decontamination required. P stages may beused in lieu of the fifth stage in the process described asmixer-settler 22 in which the plutonium values are transferred from feedalloy 18 to salt 23. The composition of salt 23 need not be, but forconvenience is, the same as the composition of salts 19. Although nobleand refractory metal fission products such as niobium, zirconium,molybdenum, technetium, ruthenium, palladium and silver are present infeed stream 18 they do not transfer with the plutonium values to salt23. Here as in the first four stages a decontamination takes place, andthe number of stages used for the noble and refractory metaldecontamination depends upon the extent of decontamination required.Part of the magnesium-copper feed alloy 18a flowing out of mixer-settler22 can be rejected to waste as stream 16w in order to prevent excessiveaccumulation of the noble and refractory metals in the feed 18 tomixer-settlers l7, and the rest of stream 180 would be recycled asstream 16 to the first mixersettler 17a. The use of waste stream 16wdepends upon the extent of the aforementioned accumulation. The last mstages of the process are captive metal stages and the last q of thelast m stages have an acceptor alloy 27 in them. The number of thesestages depends upon the specific alloy 27 or alloys employed and thedecontamination required.

The distribution of plutonium and fission products between the salt andmetal phases follows. In the first mixer-settler 17a, some of the rareearth fission products are extracted into salt 19a along with someplutonium. Feed alloy 18 then flows into mixer-settler 17b where more ofthe rare earth fission products extract into salt 19b along with someplutonium, and this process continues for each of the mixer-settlers.The salt solubility for the rare earth fission products is greater thanfor the plutonium, but some plutonium does extract into each of saltphases l9a-d. In the fifth stage, mixer-settler 22, the magnesium-copperfeed alloy 18 now depleted in most of the rare earth fission productsand also containing a diminished amount of plutonium, contacts salt 23into which extracts a majority of the plutonium values left in thealloy. The plutonium-depleted feed 18a is then recycled to the series ofmixersettlers 17 in each of which the feed 18 is mixed with the salt 19.Since feed I8 is now depleted in plutonium and equilibrium isestablished between the feed and the particular salt I9, some of theplutonium transfers out of the salt into the feed. When alloy 18 againcontacts salt 23, the salt has given up some of its plutonium values toalloy 27 and therefore salt 23 is plutonium-poor. Plutonium transfersfrom alloy 18 to salt 23. This process is continued until the requiredamount of plutonium has transferred from each and every one of themixersettlers l7a-d to salt 23 and hence to acceptor alloy 27. When therequired amount of plutonium has transferred a part of themagnesium-copper feed 18 can be rejected to waste as stream NEW, and thesalts 19 are transferred from one stage to the other as hereinbeforedescribed resulting in the rejection to waste of some of the salt asstream 20 and the addition of new salt to the process as stream 21.

The plutonium values which were transferred to salt 23 flow intomixer-settler 24 containing a captive magnesium-cadmium alloy that actsas a wash for salt 23 and provides additional decontamination for nobleand refractory metal fission products. The number of washes provided,that is the number of stages, is determined by the decontaminationdesired. The process as described shows one stage but numerous stagesmay be used with various alloys as batch or captive phases dependingupon the type of fission products present and the extent ofdecontamination desired. The magnesium-cadmium alloy 26 in mixer-settler24 may be reused for a number of subassemblies 11 but eventually isdiscarded to waste in the same manner and with the same apparatus aswill be described hereinafter for salt waste 20.

It will be appreciated that the process as heretofore described is notrestricted to the apparatus which next be described. For example, thefluid mixing process may be effected in a mixer-settler; centrifugalcontactor or any other process unit adaptable to semicontinuousoperation with multiple solvents including both captive and continuouslyflowing fluids. The apparatus in which this process may be practicedwill now be described.

Referring now to FIGS. 2 and 4, which show mixer-settlers 30a-c asrepresentative of the case where a heavy metal phase 31 is the feedsolution continuously flowing through the mixer-settlers while a lightsalt phase 32 continuously flows in each mixer-settler 30a or 30b or 30cbut remains captive and does not flow between the variousmixer-settlers. The mixersettlers 30 comprise a plurality of settlingchambers 33a-c rectangular in shape abutting each other on their longerdimensioned sides. On one of their shorter dimensioned sides, settlingchambers 33 contact a plurality of mixing chambers 34a-d, square incross section, which abut each other as well as the settling chambers.Each settling chamber 33 abuts two mixing chambers 34 and vice versa. Asimilar arrangement is shown in FIG. 3 wherein mixer-settlers 35a and bare representative of the case where a heavy metal phase, for instance,the magnesium-cadmium alloy 26 in the process hereinbefore described, isthe captive or batch phase while the continuous flow light liquid orsalt phase, for instance feed salt 23, is the feed stream. Settlingchambers 36a and b with mixing chambers 37a and b are similarly shapedand arranged as previously described for settling chambers 33 and mixingchambers 34. In each case mixing chambers 34 or 37 are below therespective settling chambers 33 and 36 so that mixed liquid in chambers34 or 37 does not leak out of chambers 34 or 37.

Reference to FIG. 4 shows a combination mixer-pump 38 used to mix heavyphase 31 and light phase 32 and pump the mixed phases from a mixingchamber 34 to a settling chamber 33. The mixer-pump 38 hereinafterdescribed will be the same whether the captive phase is a metal or asalt. As seen in FIGS. 2 and 3, a pump 39 is present in each settlingchamber 33 and 36 similar to mixer-pump 38 except that pump 39 does nothave mixing capability. Pump 39 is present to effect the periodic salttransfer from stage 17d to 17 to 17b to and from 17a to waste ashereinbefore explained.

In each mixing chamber 34 is an elongated, rotatable, hollow,cylindrical casing 40 closed at its upper end by a plate 41 and rotated(by means not shown). Casing 40 has a larger diameter upper section 42and a smaller diameter lower section 43. Fixed to the smaller diametersection 43 are two sets of mixing vanes 44 and 45 vertically displacedfrom each other, each vane 46 is generally rectangular in shape with itslonger dimensioned side parallel to the longitudinal axis of casing 40.Each set of vanes 44 and 45 comprise a plurality of vanes equally spacedabout the periphery of casing 40, six being used in the embodiment shownin the drawing.

Each mixing chamber 34, but only shown in 34d, has a set of fourvertical baffles 47 extending the length of the mixing chamber, eachpositioned on a diagonal of the mixing chamber and abutting each cornerof the chamber. The baffles 47 may have cutouts in them to accommodatevanes 46 if the vanes are otherwise too wide. in addition to thediagonal vertical baffles 47, each mixing chamber 34 has two horizontalbaffles 48 and 49 in the form of plates normal to the longitudinal axisof cylindrical casing 40, each baffle having circular apertures thereinlarger in diameter than the outer diameter of the casing. Baffle 48 ispositioned above mixing vanes 44 at the discontinuity of the largerdiameter section 42 and the smaller diameter section 43 of casing 40,and baffle 49 is positioned between the sets of vanes 44 and 45. Ahorizontal disk 49b is attached to cylindrical casing 40 at the sameelevation as baffle 49. Disk 4% extends outwardly about the samedistance as do vanes 46 which leaves a rather small annular openingbetween baffle 49 and disk 49b. More than one set of vanes 45 can beemployed if desired with additional baffles 49 and disks 4% beinglocated between each set of vanes. In addition to the above baffles is acylindrical bafile or sleeve 49a surrounding the larger diameter section42 of casing 40 extending to a point proximate but not touchinghorizontal baffle 48.

As sylindrical casing 40 rotates, liquid in the mixing chambers 34 or 37is raised and sucked upwardly through the cylinder and exits throughports 50in the cylinder wall proximate top plate 41. Collecting means 51is formed of inner and outer concentric cylinders 52 and 53,respectively, which define therebetween a trough 54 and an inner space55 between casing 40 and inner concentric cylinder 52 of the collectingmeans. Mixed liquid discharged through ports 50 is collected in trough54 and fed via pipe 56 to a settling chamber 33. Pipe 56 extends intosettling chamber 33 to prevent splashing of the mixed liquid. A shroud57 having a biased lower edge 58 may surround pump 38 interior to innerconcentric cylinder 52. Depending upon the vertical position of shroud57 with respect to exit ports 50, more or less of the mixed liquidexiting through the ports is caused to reflux to mixing chamber 34 viainner space 55 instead of flowing into trough 54 and then through pipe56 to the settling chamber 33. An inverted cup (not shown) may beaffixed to and positioned about casing 40 in trough 54 to preventsplashing, evaporization and subsequent condensation of magnesium as itexits through ports 50. Splashing is caused by contact of the liquidhaving a rotational vector with a stationary object, such as outercylinder 53.

The difference between the settling chambers 33 which contain a captiveor batch light phase 32 and the settling chambers 36 which contain acaptive or batch heavy phase 31 will now be explained.

With reference to FIGS. 2 and 4 each settling chamber 33 is adapted torecycle a light phase between it and an associated mixer chamger 34.Since the settling chambers 33a-c are identical, chamber 33a will beused as an example. As shown in the Figs, settling chamber 330 isdivided into two compartments 59 and 60 by means of a backward L-shapedbaffle 61. The battle 6] has a cross-piece 61a in contact with a longerdimensioned side of chamber 330 proximate the shorter dimensioned sideabutting mixing chamber 34a and baffle 61 extends at its other end to apoint proximate the other smaller dimensioned side of the settlingchamber. The two compartments 59 and 60 are therefore joined at theirouter ends. The baffle 61 as shown in FIG. 4, extends from the bottom ofsettling chamber 33a to a point proximate the top of the settlingchamber, and the settling chamber, itself, is higher than the mixingchamber 34. The connections between the settling chamber 330 and themixing chambers 34a and 34 b consist of vertically and horizontallyspaced apertures 62 and 63 which are aligned with correspondingapertures 64 and 65 in mixing chambers 34a and 34b, respectively. Asseen in FIG. 4, apertures 62 and 64 are nearer the top of chambers 33aand 340 than apertures 63 and 65. A conduit 66, in the form of anenclosed duct which contacts two walls of settling chamber 33a, enclosesaperture 63 and provides for flow of liquid metal 3] through apertures63 and 65 but prevents any salt 32 flow therethrough due to theproximity of the conduit with the bottom of the settling chamber.

Pipe 56 from collecting means 51 terminates in compartment 59 so thatmixed liquid from mixing chamber 340 is introduced to settling chamber330 at a point in compartment 59 separated from apertures 62 and 64 bybaffle 61. The presence of the battle 61 and cross-piece 61a preventsbackmixing of the mixed liquid from pump 38 with the separated liquidswhich flow through apertures 62 and 64.

The physical arrangement in the settling chambers 36a-b adapted torecycle a heavy phase to the associated mixing chambers 37a-b aresimilar to those described for chambers 33 and 34 identical to eachother. As shown in FIGS. 3 and 5 settling chamber 360 is divided intotwo compartments 67 and 68 by means of a backward L-shaped baffle 69.The backward L-shaped baffle 69 has a cross-piece 69a in contact with alonger dimensioned side of chamber 36 proximate the shorter dimensionedside abutting chamber 37, and baffle 69 extends at its other end to apoint proximate the other shorter dimensioned side of chamber 36.Compartments 67 and 68 are therefore joined at their outer ends likecompartments 59 and 60. An extension 70 of baffle 69 connects baffle 69with the shorter dimensioned side of settling chamber 360 abuttingmixing chamber 37a. Extension 70, as shown in FIG. 5, does not extend tothe bottom of the settling chamber 360 and with baffle 69 forms aconduit 71 similar to conduit 66. Conduit 71 provides a metal-only areain settling chamber 36a for transporting metal from compartment 68through aligned apertures 72 and 73 in chambers 36a and 37arespectively. Salt is trans ferred from compartment 68 to mixing chamber37b by means of aligned apertures 74 and 75 in settling chambers 36a andmixing chamber 36b respectively; apertures 72 and 74 are horizontallyand vertically displaced from each other similar to the displacementbetween apertures 62 and 63. However, in this case the metal transportaperture 72 is adapted to recycle the heavy phase to the mixing chamber370 from which it came rather than passing it to another mixing chamber.As in settling chamber 33a, the mixed liquid from mixing chamber 37a isintroduced into compartment 67 so as to prevent backmixing of the mixedliquid with the separated phase exiting compartment 68 through apertures72 and 74.

Combining the described process shown in FIG. I with the describedapparatus, FIGS. 2-5, feed stream 18 flows into mixing chamber 34a ofmixingsettler 17a and is mixed with salt 19a by the action of mixingvanes 44 and 45. Salt 19a is continuously circulating between mixingchamber 340 and settling chamber 33a through apertures 62 and 64.Diagonal vertical baffles 47 enhance the mixing efficiency of pump 38while the greater diameter section 42 increases the pumping capacity ofthe pump. A mixture of salt 19a and feed stream 18 is sucked upcylindrical casing 40 and discharges through ports 50 into pipe 56 andcompartment 59. As the mixed liquid flows around baffle 61 it separatesinto salt phase 19a and feed stream 18, the mass transfer of rare earthfission product values having taken place during the residence time inmixing chamber 340 and in the pump 38. The heavy metal phase (feedstream 18 in FIG. 1 and metal phase 31 in FIG. 4) flows into the conduit66 and out of settling chamber 33a through aperture 63 into mixingchamber 34b via aperture 65. Simultaneously with the metal transfer,salt 19a flows back through aperture 62 into mixing chamber 340 where itmixes with more of feed stream 18 (remember stream 18 is continuouslyflowing due to recycle stream 16) and whatever portion of the mixedliquid is refluxed due to shroud 57. This process is repeated for theidentical stages represented by mixing-settlers l'la-d.

Horizontal baffles 48 and 49 and disk 49b ensure that a greaterpercentage of the salt 19a and metal 18 remain in mixing chamber 340 fora time close to the average residence time and permits very little ofthe salt and metal to flow quickly through the mixing chamber withouthaving time to contact each other. While baffles 48 and 49 do notincrease the re sidence time in the mixing chamber 34, they do ensurethat all of the liquid feed passes through regions of intense agitationso no liquid flows directly into casing 40 without being mixed; a factvery important to the effectiveness of the mass transfer of fissionproducts between salt and metal. Sleeve 49a prevents gas entrainment inthe mixed liquids due to turbulence in the liquid caused by rotation ofa cylindrical casing 40 in chamber 34 having a square or rectangularcross section. The benefit of sleeve 49a is twofold: (l) vortexing ofthe liquid results instead of turbulence, thereby minimizing gasentrainment and increasing the mixing and pumping efficiency and (2) adefinite mixing volume is established. Since the liquid inside sleeve49a is not mixed and the liquid between the sleeve and the mixingchamber wall is quiescent, the only mixing area in mixing chamber 34 isthe space below baffle 48. The volume of this space is readilydetermined so the horse power per volume of mixed liquid is a knownquantity.

The metal feed stream 18 exits from settling chamber 33d intomixer-settler 22 where there is no captive phase. After feed stream 18is mixed with salt 23, the metal phase is in part recycled as stream 16to mixer-settler 17a and in part rejected to waste 16w, while the salt23 continues as a feed stream to the last two stages which have captivemetal phases. per FIGS. 3 and 5. The apparatus for mixer-settler 22 issimilar to that described except that the entering salt 23 and metal 18are introduced into the mixing chamber and the separated phases flow outof the settling chamber not to the mixing chamber but in one case torecycle stream 16 and in the other case to mixing chamber 24.

The last in stages, represented by mixer-settlers 24 and 25, aredescribed with reference to FIGS. 3 and wherein the heavy phase iscaptive within each mixer-settler. The mixing and separating proceduresare identical as hereinbefore described. The last q of the last m stagesare mixer-settlers having as a captive phase, a zinc-magnesium acceptoralloy for transfer of plutonium from salt 23 into the alloy. It iscontemplated that only one such stage is needed.

The mixer-settler according to the present invention lends itself to amodular construction wherein the desired number of types of mixingchambers and settling chambers are assembled to satisfy a particularpurpose. This modular construction provides great flexibility in meetingthe requirements of an extraction process by the ready addition orremoval of individual stages. It also provides easy means for service,repair or replacement of individual components.

While the foregoing description has been limited to specific shapes ofmixer-settlers and specific solvents for the masstransfer operations itshould be clear that the invention is not limited to them. Equivalentmaterials appearing in the patents cited herein and equivalentapparatus, such as cylindrical mixer-settlers or a standpipe having anopen bottom end in lieu of the conduits, are intended to be includedwithin the definition of the invention appearing in the appended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A multi-stage mixer-settler apparatus comprising a plurality fabutting mixing chamber modules and a plurality of abutting settlingchamber modules, an end wall of each of said settling chamber modulesabutting the end walls of two adjacent mixing chamber modules, thesettling chamber modules each containing a feed compartment and asettling compartment separated by a vertical L-shaped baffle, one end ofwhich terminates a short distance from the end wall of the settlingchamber module remote from the mixing chamber module and the other endof which adjoins a side wall of the settling chamber module, said baffleextending from the bottom of the settling chamber to a point proximatethe top of the settling chamber, a pumping impeller disposed in each ofthe mixing chamber modules, means for directing the effluent from thepumping impeller into the feed compartment of an abutting settlingchamber module, means for returning a captive phase from the settlingcompartment of each settling chamber module to the mixing chamber modulefrom whence it came, means for directing the other phase from thesettling compartment of each settling chamber module to the otherabutting mixing chamber module, and means for intermittentlytransferring the captive phase from each settling chamber module to anabutting settling chamber module.

2. The mixer-settler apparatus of claim 1 wherein there are apertures inthe near end wall of each settling chamber module which are aligned withapertures in the walls of both abutting mixing chamber modules, one ofthe pair of aligned apertures thus formed being at a level above the topof the heavy phase in the settling chamber module so that light phaseflows therethrough and the other pair of aligned apertures beingprotected by a baffle which extends down into the settling chambermodule to a level below the top of the heavy phase in the settlingchamber module so that heavy phase flows therethrough.

3. The mixer-settler apparatus of claim 2 wherein the pair of aperturesprotected with a baffle return the captive phase to the mixing chambermodule from whence it came, said baffle having an enclosed top and sidepanels adjoining an end wall and a side wall of the separating chambermodule, and the other pair of apertures place the settling chambermodule in flow communication with the other mixing chamber module whichabuts said settling chamber module.

4. The mixer-settler apparatus of claim 2 wherein the pair of apertureslocated at a level above the top of the heavy phase in the settlingchamber module return the captive phase to the mixing chamber modulefrom whence it came and the other pair of apertures place the settlingchamber module in communication with the other mixing chamber modulewhich abuts said settling chamber module, the vertical baffle is in theform of a backward L and the protecting baffle is a vertical extensionof said vertical baffle adjoining the near end wall of the settlingchamber module.

l II i l l

2. The mixer-settler apparatus of claim 1 wherein there are apertures inthe near end wall of each settling chamber module which are aligned withapertures in the walls of both abutting mixing chamber modules, one ofthe pair of aligned apertures thus formed being at a level above the topof the heavy phase in the settling chamber module so that light phaseflows therethrough and the other pair of aligned apertures beingprotected by a baffle which extends down into the settling chambermodule to a level below the top of the heavy phase in the settlingchamber module so that heavy phase flows therethrough.
 3. Themixer-settler apparatus of claim 2 wherein the pair of aperturesprotected with a baffle return the captive phase to the mixing chambermodule from whence it came, said baffle having an enclosed top and sidepanels adjoining an end wall and a side wall of the separating chambermodule, and the other pair of apertures place the settling chambermodule in flow communication with the other mixing chamber module whichabuts said settling chamber module.
 4. The mixer-settler apparatus ofclaim 2 wherein the pair of apertures located at a level above the topof the heavy phase in the settling chamber module return the captivephase to the mixing chamber module from whence it came and the otherpair of apertures place the settling chamber module in communicationwith the other mixing chamber module which abuts said settling chambermodule, the vertical baffle is in the form of a backward L and theprotecting baffle is a vertical extension of said vertical baffleadjoining the near end wall of the settling chamber module.